Something

//take the file name, and then get all the stuff after the last '.' (the file extension)
$file_name = $_FILES['img1']['name'];
$file_name = strrchr($file_name, ".");

//make sure file type is supported
$ext = array(".pdf", ".doc", ".xls", ".ppt");
if(!in_array($file_name,$ext))
{
	$error = "File type not supported. Supported files are ";
	foreach( $ext as $exts ){
	$error .= "$exts ";
	}
	die ("$error");
}